Method and device for determining elasticity of cement stone utilized in well cementing of oil-gas well

Abstract

The invention provides a method and device for measuring the elasticity of hardened cement for cementing of oil-gas wells. The measurement method comprises: determining the loading and unloading rates of the hardened cement; determining the maximum loading on the hardened cement; determining the experimental temperature and the experimental pressure of the hardened cement; establishing a stress-strain curve for the hardened cement; and describing the elasticity of the hardened cement with the degree of strain recovery of the hardened cement in different cycles, and describing the mechanic integrity of the hardened cement with the degree of damage to the hardened cement in different cycles. The invention further provides a device for measuring the elasticity of hardened cement for cementing of oil-gas wells. The measurement method and device of the present invention provide a universal comparing platform for research on hardened cement modification as well as examination of domestic and foreign special cement slurry systems, which is of great significance in evaluation of well hole integrity and well life.

The invention claimed is: 
     
       1. A method for measuring the elasticity and mechanical integrity of hardened cement for cementing of oil-gas wells, the method comprising the steps of:
 i) determining the loading and unloading rates of the hardened cement, according to the force applied on the hardened cement and the duration required for the loading and unloading of the force, in various engineering operations; 
 ii) establishing a normal stress-strain curve, and determining the maximum loading on the hardened cement by comparing the data of the force applied on the hardened cement in the various engineering operations to the normal stress-strain curve; 
 iii) determining the experimental temperature and the experimental pressure, and establishing a stress-strain curve for the hardened cement by conducting a multi-cycle tri-axial stress test on the hardened cement at the experimental temperature and the experimental pressure based on the determined loading rate, unloading rate and maximum loading of the hardened cement; and 
 obtaining a quantitative evaluation of the elasticity of the hardened cement with the degree of strain recovery of the hardened cement in different cycles; and 
 iv) obtaining a qualitative evaluation of the mechanic integrity of the hardened cement by conducting a multi-cycle mechanic test by a testing method using alternating loadings. 
 
     
     
       2. The method according to  claim 1 , characterized by further comprising a step of preparing an experimental sample of hardened cement, prior to the measurement of the elasticity of the hardened cement for cementing of oil-gas wells, the preparing comprising:
 preparing an experimental cement slurry, setting and hardening the cement slurry into hardened cement by curing the cement slurry under a simulated temperature and pressure condition for hardened cement for a designated period of time according to the downhole environment surrounding the hardened cement, and processing the hardened cement to a standard core size to obtain the sample of hardened cement. 
 
     
     
       3. The method according to  claim 1 , characterized in that the normal stress-strain curve is established through a tri-axial stress test. 
     
     
       4. The method according to  claim 1 , characterized in that the determining the maximum loading on the hardened cement comprises the step of:
 determining the average value of the maximum strains of the hardened cement, and determining the maximum loading as the stress value corresponding to the maximum strain according to the normal stress-strain curve. 
 
     
     
       5. The method according to  claim 1 , characterized in that the experimental temperature and the experimental pressure are determined in accordance with the downhole depth of the hardened cement. 
     
     
       6. The method for according to  claim 1 , characterized in that the degree of strain recovery of the hardened cement in different cycles is determined according to the following equation:
   (maximum strain upon loading−minimum strain upon unloading)/maximum strain upon loading.
 
 
     
     
       7. The method according to  claim 1 , characterized in that for the qualitative evaluation of the mechanic integrity of the hardened cement,
 if the hardened cement shows microcracks or breaks, the hardened cement cannot withstand the mechanical impacts from various subsequent engineering operations, indicating lack of mechanic integrity; or 
 if the hardened cement does not show microcracks or break, the hardened cement can withstand the mechanical impacts from various subsequent engineering operations, indicating mechanic integrity.